1. Field of the Invention
The present invention relates to a bioinformation acquisition apparatus using light and in particular to technology for enhancing optical energy use efficiency by re-irradiating a living body with a flux of light that has diffused through and been emitted from a living body.
2. Description of the Related Art
Research and development have been done for visualizing information in a living body by using near infrared rays because infrared light is noninvasive and is transmitted well through a living body, i.e., a test object. In vivo material such as water, fat or hemoglobin present in a blood vessel has a peculiar spectrum in the near-infrared wavelength range. Much study, therefore, has directed to visualizing a spatial coefficient distribution of absorption originating from those components to acquire functional information in a living body.
A method for detecting optical absorption in a living body includes diffuse measurement (DOT: Diffuse Optical Tomography), in which a flux of light that has been transmitted through a living body is directly measured, and photoacoustic measurement (PAT: Photo Acoustic Tomography), using the photoacoustic effect.
In PAT, a living body is irradiated with pulsed light generated by a light source, and an acoustical wave, produced when the living body absorbs optical energy from the pulsed light which propagates and is diffused therein, is measured. That is, an elastic wave produced when a test site expands instantaneously by absorbing radiated optical energy is received by a transducer, taking advantage of a difference in absorption coefficient of optical energy between the test site such as a tumor and tissue other than the test site. This detection signal can be analyzed and processed, providing an optical characteristics distribution in a living body, particularly an optical energy absorption density distribution. An ultrasonic wave has high straightness in a living body compared with light, and hence can be observed with a high spatial resolution by using a photoacoustic measurement apparatus for detecting ultrasonic waves.
U.S. Patent Application Publication No. 2006/0184042 proposes an apparatus as a bioinformation imaging apparatus using the photoacoustic effect. The apparatus disclosed in that document is adapted so that light radiated to a desired area in a living body is collected by a lens having a light-collecting function, and a photoacoustic signal is detected, while making efficient use of optical energy.